There have been proposed a variety of methods for the manufacture of microcapsules for a broad range of uses such as those mentioned above. Representative are the method comprising the use of a urea-formaldehyde copolymer for the outer layer (Japanese Patent Publication No. 54-16949), the method comprising the use of a urea-formaldehyde copolymer or melamine-formaldehyde copolymer as the shell component (Japanese Kokai Patent Publication No.62-1451), and the method comprising the use of a urea-formaldehyde copolymer, melamine-formaldehyde copolymer or urea-melamine-formaldehyde terpolymer for the outer layer (Japanese Kokai Patent Application No.63-134048).
However, since the surface of the shell structure of microcapsules manufactured by any of these methods is so strongly hydrophilic that they cannot be efficiently dispersed in an oil-soluble resin or an organic medium in which such an oil-soluble resin has been dissolved, with the result that these microcapsules are not suited for ink, coating composition and like applications.
In order to recover the microcapsules so manufactured in particulate form from an aqueous water-soluble polymer system in which they are dispersed, a spray-drying technique is generally employed but this technique involves the use of heat, resulting in an additional energy cost.
Another disadvantage of this technique is that the heat treatment tends to cause degradation of both the shell and inner material of the microcapsules.
Moreover, the production line employing the technique must be provided with a heating chamber for the heat treatment and this entails use of larger production equipment.
A still another drawback of the spray-drying technique is that a large amount of formaldehyde derived from the starting comonomer remains in the microcapsules manufactured and constitutes a source of unpleasant smell and presents a health hazard as well.